The miniaturization of integrated circuits has been underway ever since the first demonstration of an integrated circuit. Using dynamic random access memory (DRAM) as a benchmark, current expectations of device generations, dates of peak production, and lithography critical dimension are: (4 Mb, 1994, 0.8 .mu.m); (16 Mb, 1997, 0.5 .mu.m); (64 Mb, 1999, 0.35 .mu..mu.); (256 Mb, 2003, 0.25 .mu.m) and (1 Gb, 2006, 0.15 .mu.m) [projections from R. J. Kopp, Semiconductor International, 15, 34-41 (1992)]. In the industry news section in the same issue of this trade magazine (page 11), there is a report of a MICROTECH 2000 workshop cosponsored by the National Advisory Committee on Semiconductors (NACS) and the Office of Science and Technology Policy (OSTP). The reported recommendation relative to lithography is: "An experimental lithography capability that can print features of 0.10 to 0.15 .mu.m will be required by 1994 in sufficient volumes to allow essential process and manufacturing equipment development. This need may require new electron-beam mask or direct wafer writing tools, or a capability in advanced X-Ray or phase-shift optical lithography. Research and development for several lithography alternatives will have to be supported for the next several few years to determine what system is best suited for production."
Imaging optical lithography, in which a mask image is projected onto a photoresist layer on the wafer, dominates today's manufacturing. Two equations describing the optical diffraction of the optical system determine the characteristics of the image. The minimum resolution, r, is proportional to the lens numerical aperture, or EQU r.about.1/NA
and the depth of focus (DOF) EQU DOF.about.1/(NA).sup.2
where 1 is the wavelength and NA the lens numerical aperture. These simple equations point out some of the difficulties in extending optical lithography to the extreme submicrometer regime, i.e., about 0.1 .mu.m or 100 nm. Refractive optics are available only up to approximately 200 nm; at shorter wavelengths almost all materials become strongly absorptive and unusable. There are several efforts underway to use reflective optics at short wavelengths. However, there remain significant materials problems, particularly at X-Ray wavelengths and the NAs of these systems are significantly lower than for refractive systems, giving away some of the wavelength advantage for imaging small areas.
Considerable interest and attention have been given to new X-ray lenses based on grazing incidence filamentary propagation through hollow "waveguides." This remains a difficult problem without a demonstration of a high-efficiency, high numerical aperture, manufacturable lens with a field-of-view that can accommodate today's growing field sizes. From the experience of longer wavelength optical lithography using refractive lenses, the optical train can easily be the most complex and expensive part of a lithography tool.
The progression to short wavelengths to improve the minimum resolution carries a concomitant penalty in the reduction of the depth-of-focus (DOF). This has motivated efforts at multilayer resists with strong absorption layers, as well as efforts at improved planarization of circuits to eliminate topographic variations that would cause different parts of the circuit to image at different heights. This small DOF is a major concern for submicrometer lithography.
Briefly, major issues facing extension of conventional lithography to the extreme submicron regime (0.1 .mu.m) include: source technology (issues are uniformity, spectral bandwidth, repeatability, reliability, etc.); the imaging system (again refractive optics become impossible below .about.200 nm and reflective optics have inherently smaller numerical apertures); the mask technology (there are significant issues related to vibration, heating and distortion in X-Ray masks which must be fabricated on pellicle substrates because of the strong X-Ray absorption of most materials); and the resist technology.
For many years periodic line and space gratings in the extreme submicron range have been fabricated by use of two interfering coherent beams. For two beams incident at angles .theta. and -.theta. to the surface normal, the period of the interference pattern is .lambda./(2 sin .theta.). For readily available wavelengths (361-nm Ar-ion laser) and angles (.theta..about.75.degree.) this gives a period as small as 187 nm. The resulting grating pattern is a periodic line and space array; the critical dimensions of the lines are adjustable using nonlinearities in the expose and develop processes to roughly 1/3 of this dimension or 60 nm.